European patent publication 0,177,517 discloses a plasma CVD process (Plasma-Chemical-Vapor-Deposition process) for applying thin transparent coatings to the surface of optical elements. This is a method for producing gradient-type scratch protective coatings especially on plastic substrates wherein the composition of the coating is so changed that the substrate side of the coating exhibits excellent adherence and the side of the substrate exposed to air exhibits high hardness. The above is achieved by continuously increasing the portion of a hardness-increasing component (preferably oxygen) to the organic coating gas (a silicon-organic compound).
The change of the layer composition comprises essentially a continuous transition from organic to inorganic in the direction of layer growth.
This method has the disadvantage that mass flows, which change as a function of time, can be adjusted to produce layers only with a process stability less than mass flows which are constant as a function of time. The layers are gradient forming in their composition. Conventionally used mass flow controllers are very sluggish with a set time of at least 0.5 seconds and therefore they are not suitable to produce layers having steep gradients (for example, 1% concentration change per nm of layer thickness) and/or very thin gradient layers (for example, an optical .lambda./4 layer of approximately 85 nm) wherein concentration changes must take place in the second range or lower for the usual deposition rates of 500 nm/min.
The production of such layers would be in principle possible by correspondingly reducing the deposition rate. However, this requires longer coating times and special controllers for low mass flows which are very cost intensive. Accordingly, this procedure is very disadvantageous economically. In principle, it would be possible, for example, to correspondingly thin the coating gas to obtain reduced coating rates. However, this can result in unwanted effects on other layer characteristics such as the refractive index. In this connection, reference can be made to the article of Hernandez et al entitled "Kinetics and Compositional Dependence on the Microwave Power and SiH.sub.4 /N.sub.2 Flow Ratio of Silicon Nitride Deposited by Electron Cyclotron Resonance Plasmas" published in the J. Electrochem. Soc., Volume 141, No. 11, November 1994, pages 3234 to 3237.
An article by Wertheimer et al entitled "Advances in Basic and Applied Aspects of Microwave Plasma Polymerization" published in Thin Solid Films, Volume 115 (1984), pages 109 to 124, discloses that, in plasma polymerization of HMDSO (hexamethyldisiloxane), the following occurs with increasing power and/or substrate temperature: the structure of the deposited layer changes, the content as to organic constituents significantly decreases, the etch rate of the layer increases and the refractive index increases with increasing substrate temperature.
The above effect is utilized in the method disclosed in U.S. Pat. No. 5,217,749 for producing a refractive-index gradient layer. In this method, layers are polymerized with a PCVD method from the monomer vapor of an organic compound. A refractive index gradient in the direction of layer growth is generated only by continuously changing the process power level during the coating process.
In this method, several mechanisms however operate so that a targeted change of the layer composition becomes problematical.
On the one hand, the production of the layers takes place from coating gases wherein fresh gas and residual gas from the previous reaction are mixed with each other in an undefined manner. The composition of the layer produced and the layer characteristics dependent thereon can change in an undefined manner because of this situation when any one of the coating parameters is changed. On the other hand, when increasing the power level, the substrate temperature increases whereby the unwanted change of the parameter, substrate temperature, is produced by the wanted change of the parameter, power. Furthermore, a reduction in species capable of reaction takes place in the direction of the flow of the supplied fresh gas over the substrate so that the layer characteristics are different in this direction. The second mechanism furthermore causes the situation that heat-sensitive substrates cannot be coated.
Furthermore, it is disadvantageous that the coating rate of approximately 4 nm/min is very low and can be increased apparently only at the cost of lateral uniformity for the given pressure of 60 mTorr and the RF-power by increasing the mass flow of the coating gas.